How do scientists measure the size of the earth?

Erastotenes estimated the size of the Earth for the first time 2200 years ago, and subsequent estimates were based on methodological improvements.

“Eratosthenes” on https://tutor.hix.ai

Using the current unit measures, he calculated that the distance between Aswan and Alexandria was approximately 880 km. On the day of the Summer Solstice, which falls around June 21, the Sun was completely in zenith (above our heads) in Aswan, but on the same day, an angle of approximately 7 degrees was measured in Alexandria between the zenith and the position of the Sun (using the shadow of a vertical pole). This led him to conclude that the 880 km corresponded to 7º degrees of the Earth's circumference, simply by applying the rule of three.

Scientists use a variety of techniques, such as triangulation, satellite observations, and gravitational pull measurements, to determine the size of the Earth.